It is known to heat the steering wheel of a vehicle for the comfort of the driver. Known systems for heating steering wheels typically comprise a heater mat which includes one or more heater elements, a temperature sensor and electronic controls. The heater elements are arranged in series along a single electrical circuit. Typically, the whole rim of the steering wheel is wrapped with a heater mat, to provide a comfortable temperature on the entire surface of the rim.
A clockspring is a rotary electrical connector which allows a vehicle's steering wheel to turn while still making an electrical connection between the steering wheel airbag and/or the vehicle's horn and other devices and the vehicle's electrical systems. The clockspring is located between the steering wheel and the steering column.
The limiting factor in heating up a steering wheel in a short period of time is the maximum electrical energy (the product of the voltage and current) which can flow through the clockspring. For a fixed value of maximum electrical energy, the greater the area being heated the longer the heating will take. As a result, known heated steering wheel systems suffer from a relatively long heating up time.
In other known systems, this problem has been addressed by only partially covering the steering wheel rim with heating elements. Typically, it is the 3 o'clock and 9 o'clock regions of the steering wheel that are heated as these are the portions of the steering wheel that are most contacted by the driver. However, the other portions (at the 6 o'clock and 12 o'clock regions) are not heated at all and the driver does contact these portions, such as during a turning maneuver. This is inconvenient for the driver and may even encourage a poor style of steering wheel handling.
It is desirable to provide an alternative heated steering wheel which provides at least some heating of the entire rim of the steering wheel while heating at least the most used portions of the rim in a practically short length of time.
A thermistor is a known type of resistor whose resistance varies significantly with temperature. For a positive temperature coefficient (PTC) thermistor, the resistance is typically low at ambient temperatures and increases with increasing ambient temperature. For a negative temperature coefficient (NTC) thermistor, the resistance is high at ambient temperatures and decreases with increasing ambient temperature.